When driving a resonant load with a high and low side half-bridge driver circuit, it is necessary to fulfill zero-voltage switching. This ensures smooth AC currents and voltages and provides a continuous uninterrupted inductor current. Should non zero-voltage switching occur while driving a fluorescent lamp with a resonant output stage (FIG. 1), high current spikes appear in the half-bridge switches, which can exceed the maximum current ratings of the switches and/or the resulting power losses in the switches can cause the switches to thermally destruct.
Non zero-voltage switching can occur due to one or both lamp filaments breaking resulting in an open-circuit, or a normal running lamp but a decreasing DC bus voltage. In each case, the half-bridge output voltage, V.sub.S (FIG. 2), must commutate to zero volts before the lower switch turns on or must commutate to the DC bus voltage before the upper switch turns on. If no lamp is present, no inductor current flows to commutate the capacitance from V.sub.S to ground due to the switch and (if present) the snubber capacitor C1. The circuit of the present invention senses the resulting current spike and turns both half-bridge switches off if spike exceeds a predetermined value.